Polymeric photoinitiators

ABSTRACT

UV curable compositions, polymeric photoinitiators and precursors therefor are described.

This is a divisional of U.S. application Ser. No. 10/465,473, filed Jun.18, 2003 now U.S. Pat. No. 7,157,535, which application claims thebenefit of the earlier Jun. 19, 2002 filing date of U.S. provisionalapplication No. 60/389,972.

FIELD OF THE INVENTION

The invention relates to polymeric photoinitiator compositions,precursors useful in the preparation of such polymeric photoinitiatorsand to the use of these polymers in, e.g., UV curable adhesives, UVcurable coating compositions and UV curable encapsulants.

BACKGROUND OF THE INVENTION

The primary function of a photoinitiator is to generate polymerizationinitiating radicals when the photoinitiator is irradiated withultraviolet (UV) light. Photoinitiators are classified into “Type I” (orphotocleavage) photoinitiators and “Type II” (or H-abstraction)photoinitiators according to the pathways by which the effectiveinitiating radicals are generated.

In contrast to photocleavage photoinitiators which are decomposed by UVlight directly into radicals which are effective in initiatingpolymerization, H-abstraction photoinitiators require a hydrogen donor,or more generally a source of abstractable hydrogens in order togenerate radicals that are effective in initiating polymerization. Theprocess of H-abstraction is usually a bimolecular reaction requiring theencounter of a photoinitiator and a hydrogen-donor. Any source ofabstractable hydrogens may be useful (e.g., any structure that yields astable radical after H-abstraction may serve as a “H donor”) and suchsources include amines, thiols, unsaturated rubbers such aspolybutadiene or polyisoprene, and alcohols.

The basic photochemistry and photophysics of both α-cleavage (Type I)and H-abstraction (Type II) photoinitiators has been well studied andutilized industrially in UV curable systems (see (a) Cowan, D. O.;Drisko, R. L. Elements of Organic Photochemistry, 1976, Plenum Press,chapters 3 and 4. (b) Turro, N. J. Modern Molecular Photochemistry,1991, University Science Books, chapters 7, 10, and 13.). One wellrecognized problem with the use of UV curable systems for coating andadhesive applications is the fate of the photo-by products created bythe curing process. In the case of typical α-cleavage typephotoinitiators, the production of benzaldehyde (and often relatedcompounds) is often a significant concern from both a toxicity andproduct odor standpoint. Such concerns become especially important whenthe use of radiation curable materials is considered for applicationsthat involve skin or food contact. Various effective approaches havebeen taken to reduce the odor and extractable by-product content of UVcurable materials. One approach has been the use of copolymerizeable orpolymeric photoinitiators which are chemically incorporated into thecured polymeric matrix as opposed to remaining in the irradiatedmaterial as a small molecule (see (a) Fouassier, J. P.; Rabek, J. F.,Eds. Radiation Curing in Science and Technology, 1993, Elsevier Appl.Sci., vol. 2, 283-321. (b) Fouassier, J. P. Photoinitiation,Photopolymerization and Photocuring, Fundamentals and Applications,1995, Hanser Publishers, 71-73.). Unfortunately, when utilizingα-cleavage photoinitiators at least one of the cleavage by-productsstill remains as a small molecule even if the other fragment isincorporated into a polymeric component of the system. Thus, althoughextractable and odorous by-products can be reduced through the use ofpolymeric or polymerizeable Type I photoinitiators, they are noteliminated entirely.

The use of polymerizeable or polymeric H-abstraction typephotoinitiators, in principle, presents the possibility of creating asystem with zero extractable components related to the photoinitiationsystem. Various groups have presented systems based upon poly(vinylbenzophenone) and its copolymers or polymers derived from acrylatedbenzophenone derivatives (see (a) David, C.; Demarteu, W.; Geuskens, G.Polymer, 1969, 10, 21-27. (b) Carlini, C.; Ciardelli, F.; Donati, D.;Gurzoni, F. Polymer, 1983, 24, 599-606.). The direct use of acrylatedbenzophenones has also been disclosed (U.S. Pat. No. 3,429,852).Unfortunately, these Type II systems often suffer from issues related tophotoefficiency relative to analogous small molecule photoinitationsystems.

It is notable that, in products requiring very low extractable levels orodor, any copolymerizeable photoinitiator that does not fully react intothe growing polymer network will remain in the final product as a smallmolecule, creating many of the same problems that photoinitiatorfragments introduce. Thus, it is often most desirable to use polymericphotoinitiators as opposed to polymerizeable photoinitiators. A keypractical issue when utilizing polymeric photoinitiators is theircompatibility with the resins systems in which they are to be used. Anadditional requirement of such polymeric photoinitiators, particularlythose intended for use in hot melt adhesives and coatings, is that thatthey be thermally stable at the application temperature. The polymericType II photoinitiators known in the prior art fail to meet one or moreof the requirements and needs noted above.

There continues to be a need in the art for improved H-abstractionphotoinitiators useful in the manufacture of radiation curable adhesivesand coating formulations. In particular, polymer-bound H-abstractionphotoinitiators are needed to produce low odor products with fewer (orno) inherent extractable photochemical by-products. The currentinvention fulfils this need.

SUMMARY OF THE INVENTION

One aspect of the invention is directed to precursor compounds useful inthe preparation of H-abstraction class photoinitiators, in particularpolymeric H-abstraction class photoinitiators, more specifically rubberbound H-abstraction class photoinitiators. In a preferred embodiment theprecursor is a SiH-functional benzophenone derivative which may be usedin the preparation of polymeric benzophenone photoinitiators.

Another aspect of the invention is directed to rubber-bound polymericH-abstraction class photoinitiators. In one embodiment the rubber-boundpolymeric H-abstraction class photoinitiator is astyrene-butadiene-styrene (SBS)-bound polymeric H-abstraction classphotoinitiator. In a particularly preferred embodiment the rubber-boundpolymeric H-abstraction class photoinitiator is a poly(butadiene)-boundpolymeric H-abstraction class photoinitiator. Both amide-linked,sulfide-linked, and silane-linked polymeric H-abstraction photoinitiatorcompositions are encompassed by the invention.

Still another aspect of the invention is directed to UV curablecompositions comprising a polymeric H-abstraction photoinitiator. In apreferred embodiment the UV composition is a UV curable hot meltpressure sensitive adhesive. Adhesive and coating formulations based on,but not limited to, rubber, thiol-ene, maleimide and acrylate baseresins are encompassed. A particularly preferred adhesive is a SBS orSIS rubber-based UV curable pressure sensitive adhesive comprising apoly(butadiene)-bound polymeric H-abstraction class photoinitiator.

Yet another aspect of the invention is directed to radiation cured hotmelt pressure sensitive adhesives and radiation cured coatingcompositions, and to articles of manufacture comprising the curedadhesive and/or coating compositions.

A further aspect of the invention is directed to the use of the novelpolymer-bound photoinitiators in UV curable coatings and adhesives inthe same manner which small molecule Type II photoinitiators are oftenused. Preferred applications include thiol-ene, acrylate and maleimidebased UV curable compositions.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 shows the photoDSC analysis of a thiol-ene photopolymerizationwith and without the polymeric photoinitiator of the invention

FIG. 2 shows the photoDSC analysis (300 nm cutoff filter) of a thiol-enephotopolymerization with and without the polymeric photoinitiator of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

All documents cited herein are incorporated in their entireties byreference.

The current invention provides polymeric H-abstraction classphotoinitiators, in particular rubber-bound H-abstraction photoinitiatorcompositions, and adhesives, coating compositions and the likecomprising such polymeric photoinitiators.

The polymeric H-abstraction class photoinitiators of the invention areprepared from precursors comprising a chromophore linked through aspacer unit to a functional group. Silane-functional,mercaptan-functional and amine-functional chromophores are encompassed.The unit between the functional group and the chromophore can be anyorganic moiety, preferably straight and branched chain alkyl or aryl,possibly including (but not limited to) heteroatoms such as O, S, N andSi. The spacer unit can also possess pendant heteroatom-containingsubstituents such as, but not limited to, alkoxy, mercaptoalkyl, aminoand alkylamino. These functional precursors are subsequently covalentlyattached to unsaturated small molecules or polymers to produce theinventive polymeric photoinitiator molecules.

The functionalized chromophores of the invention are attached to smallmolecules or polymers that contain groups that are reactive with thefunctional group present on the chromophore.

Small molecules are defined are being non-polymeric materials. Forexample, SiH-functional chromophores may be reacted via a hydrosilationreaction with unsaturated small molecules such as bisphenol A diallylether, trimethylolpropoane triallyl ether or multifunctional vinylethers such as cyclohexanedimethylol divinyl ether to form smallmolecule photoinitiators. These may be mono- or multifunctionaldepending on the choice of unsaturated small molecule.

As previously noted herein, it is often preferable/advantageous to graftone or more of the functional chromophores to a polymeric material toproduce a polymeric photoinitiator. For example, SiH-functionalchromophores may be grafted through hydrosilation to an unsaturatedpolymer such as poly(butadiene). The resulting polymeric photoinitiatorswill often contain more than one covalently linked chromophore perpolymer chain, and the average number of chromophores per chain can becontrolled through stoichiometry. In cases where the polymericphotoinitiator may also participate in UV crosslinking reactions as wellas acting as a photoinitiator, it is often preferable that the polymericphotoinitiator contain more than two chromophore groups.

The light absorbing chromophores used in the photoinitiator system ofthe invention are chosen to match as closely as possible the emissionbands of the light source. Useful chromophores include compounds whichundergo H-abstraction photochemistry including but not limited tobenzophenone and related aromatic ketones such as xanthone,thioxanthone, 4,4′-bis(N,N′-dimethylamino)benzophenone, benzil,quinones, quinoline, anthroquinone, fluorene, acetophenone, xanthone,phenanthrene and fluorenone. A reasonably comprehensive list of usefulchromophores and photophysical data for such can be found in the PolymerHandbook, Brandrup J.; Immergut, E. H.; Grulke, E. A.; Eds., John Wiley& Sons, Inc., II/169, “Photopolymerization Reactions”, Fouassier, J. P.;1999.

Preferred SiH-functional chromophores for use in the practice of theinvention are SiH-functional diarylketone having the followingstructure:

wherein

Ar is an aryl ketone moeity,

R is independently a linear or branched alkyl group, cycloalkyl,alkylenoxy, alkenyl or aryl group, an alkyl group containing aheteroatom, or H,

R¹ is independently a linear or branched alkyl group, cycloalkyl,alkylenoxy, alkenyl or aryl group, or an alkyl group containing aheteroatom or a carbonyl group,

X is O, NR, S, PR, or SiR₂,

n=0-2, and

m=0 or 1.

Particularly preferred are SiH-functional arylketones wherein Ar isselected from the group consisting of

wherein

R² is independently a linear or branched alkyl group, cycloalkyl,alkylenoxy, alkenyl or aryl group, an alkyl group containing aheteroatom, a carbonyl group, H, OR, NR₂, SR, F, Cl, Br or I.

In one preferred embodiment, the precursor which can be used to preparepolymeric benzophenone photoinitiators is a SiH-functional benzophenonederivative having the structure shown in formula IA.

In another embodiment of the invention, the precursor which can be usedto prepare polymeric thioxanthane photoinitiators is amercapto-functional thioxanthane derivative having the structure shownin formula IB.

Precursor compounds IA and IB of the invention may be prepared asdescribed in Examples 2 and 6, respectively.

The precursor compounds of the invention are grafted to a polymerbackbone to form polymer bound Type II photoinitiators. Any polymer withresidual unsaturation or that has been maleinized (reacted with maleicanhydride) can be used in the practice of the invention. Unsaturatedpolymers that can be used in the practice of the invention include butare not limited to styrene-butadiene-styrene (SBS),styrene-isoprene-styrene (SIS), poly(butadiene) (pBD), randomstyrene-butadiene-styrene rubber (SBR),ethylene-propylene-dicyclopentadiene (EPDM) and acrylate polymerscontaining pendent or backbone unsaturation. Maleated base polymersinclude: acrylates, polybutadiene, polyisoprene, SIS, SBS,styrene-b-ethylene/butylene-b-stryrene,styrene-b-ehtylene/propylene-b-styrene, or polyisobutylene.

One preferred photoinitiator is a siloxane-linked photoinitiator formedby reacting a SiH-functional diaryl ketone with an unsaturated polymer(Q) is represented by the structure:

wherein

Ar, is as defined as above,

Q is a small molecule or polymer,

R is independently a linear or branched alkyl group, cycloalkyl,alkylenoxy, alkenyl or aryl group, an alkyl group containing aheteroatom, or H,

R¹ is independently a linear or branched alkyl group, cycloalkyl,alkylenoxy, alkenyl or aryl group, or an alkyl group containing aheteroatom or a carbonyl group,

X is O, NR, S, PR, or SiR₂,

n=0-2,

m=0 or 1, and

y=1-100.

In some systems, type II photoinitiators can participate incrosslinking, in which case y is preferably 2 or more so that it is amultifunctional material.

Preferred sulfide-linked photoinitiators are represented by thefollowing structure:

wherein

Q and Ar are as defined above,

R is independently a linear or branched alkyl group, cycloalkyl,alkylenoxy, alkenyl or aryl group, an alkyl group containing aheteroatom or a carbonyl group,

X is O, NR, S, PR, or SiR₂,

m=0 or 1, and

y=1-100.

Preferred amide-linked photoinitiators are represented by the followingstructure:

wherein

Q and Ar are as defined above,

R is independently a linear or branched alkyl group, cycloalkyl,alkylenoxy, alkenyl or aryl group, an alkyl group containing aheteroatom or a carbonyl group,

R¹ is independently a linear or branched alkyl group, cycloalkyl,alkylenoxy, alkenyl or aryl group, an alkyl group containing aheteroatom, a carbonyl group or H.

X is O, NR, S, PR, or SiR₂,

m=0 or 1,

n=0 or 1, and

y=1-100.

The presence of the chromophores in the polymeric photoinitiators of theinvention renders them sensitive to ultraviolet and/or visibleirradiation and thus capable of initiating and/or participating incrosslinking upon exposure to such a source of light.

Polymeric photoinitiators of the invention prepared by bonding theprecursor compounds IA and IB to poly(butadiene) are shown in formulasIIA and IIB, respectively.

Polymeric photoinitiator compounds IIA and IIB of the invention may beprepared as described in Examples 3 and 7, respectively.

The polymeric photoinitiators of the invention may be used to prepare awide variety of radiation curable materials including pressure sensitivehot melt adhesives and coating compositions. The use of the term coatingcompositions is used broadly herein to mean decorative and abrasionresistant coatings, lacquers, fiber reinforced composites,microelectronic encapsulations, die-attach, fiber optic coatings,molding compounds, UV-set structural resins and the like.

Photocurable compositions contemplated for use include compositionscomprising a siloxane-linked photoinitiator derived from the reaction ofa SiH-functional diarylketone and an unsaturated compound, asulfide-linked photoinitiator derived from the reaction of aSH-functional diarylketone and an unsaturated compound and/or anamide-linked photoinitiator derived from the reaction of anamino-functional diarylketone and a compound containing either ananhydride or a carboxylic acid functionality. Included are thiol-enephotocurable compositions comprising a multifunctional thiol, amultifunctional olefin, and the photoinitiators.

In a radiation curable compositions, crosslinking occurs by exposure toactinic and/or ionizing radiation. The term “radiation” is used hereinto include actinic radiation such as ultraviolet radiation and ionizingradiation created by the emission of electrons or highly acceleratednuclear particles such as neutrons, alpha-particles etc.

Relative to typical commercially available photoinitiators, thepolymeric photoinitiators of the invention are thermally andhydrolytically stable before and after UV cure, exhibit reduced odorafter cure in UV curable compositions, exhibit reduced extractables inUV curable compositions, and efficiently UV cure thick films.

Pressure sensitive hot melt adhesive compositions of the type describedin, e.g., U.S. Pat. Nos. 4,820,746, 4,948,825, 5,093,406, 5,104,921,5,115,008, 5,135,978, 5,166,226, 5,302,649, 5,614,577 and 5,804,663 maybe prepared using the polymeric photoinitiators of the invention.

In the practice of the invention, any base resins suitable for use informulating adhesives and coating compositions, as are well known tothose skilled in the art, may be used in the practice of the invention.Useful polymers include amorphous polyolefins, ethylene-containingpolymers and rubbery block copolymers, as well as blends thereof.Systems, based on acrylate, epoxide, siloxane, styryloxy, vinyl etherand other monomers, oligomers, or prepolymers such as polyimides andcyanate ester resins and/or polymers and hybrids thereof, may be used.The compositions may be selected from liquid or solid olefinicallyunsaturated systems, such as acrylates, methacrylates, maleimides,styrenics, maleate esters, fumarate esters, unsaturated polyesterresins, alkyl resins, polyisoprene, polybutadiene and thiol-enecompositions. Polymers such polyisoprene or polybutadiene, or theirrandom or block copolymers with styrene may be used with or withoutpartial hydrogenation of the diolefin. To increase reactivity, suchpolymers with no to full hydrogenation of the diolefin can be acrylatedvia grafting of maleic anhydride followed by the ring opening reactionwith a hydroxy acrylate. Such materials are available under thetradename Ricacryl from Sartomer and from Kuraray Chemical as UC Resins.Functional oligomeric or polymeric resins based on backbone structuressuch as silicones, polyesters and urethanes, such as acrylate,methacrylate, maleimide or vinyl-terminated resins may also be utilized.Particularly useful rubber-based pressure sensitive hot melt adhesiveswill preferably contain at least one block derived from a vinyl aromaticmonomer and at least one block derived from butadiene or isoprene, suchas a tackified mixture of styrene-butadiene-styrene block copolymer anda styrene-butadiene block copolymer or a tackified mixture ofstyrene-isoprene-styrene block copolymer. It is to be understood thatrubber-rubber blends as well as acrylic/rubber blends are encompassed asare acrylic-rubber hybrids (e.g., an acrylic polymer grafted with anethylene-butylene rubber macromer).

The radiation curable hot melt adhesives used in the practice of theinvention may be formulated to be pressure sensitive, semi-pressuresensitive, or non-pressure sensitive. As used herein, the term“pressure-sensitive adhesive” refers to a viscoelastic material whichadheres instantaneously to most substrates with the application ofslight pressure and remains permanently tacky. A polymer is apressure-sensitive adhesive within the meaning of the term as usedherein if it has the properties of a pressure-sensitive adhesive per seor functions as a pressure-sensitive adhesive by admixture withtackifiers, plasticizers or other additives. A semi-pressure sensitiveadhesive is one which temporarily possesses sufficient tack topermanently bond the substrate. After this time the adhesive is stillpermanently tacky but not enough to create a strong bond. Semi-pressuresensitive adhesives are typically used as regular hot melts. That is thebond is made while the adhesive is still molten. The fact that bondabletack extends through the solidification stage creates the opportunity tobond over a wide process range. One cannot coat these types of adhesivesand create a bond with them at a later date as with a true pressuresensitive.

Non-limiting examples of polyolefin polymers useful in the practice ofthe invention include ethylene copolymer as well as blends thereof. Theterm ethylene copolymer, as used herein, refers to homopolymers,copolymers and terpolymers of ethylene. Examples of ethylene copolymersinclude copolymers with one or more polar monomers which cancopolymerize with ethylene, such as vinyl acetate or other vinyl estersof monocarboxylic acids, or acrylic or methacrylic acid or their esterswith methanol, ethanol or other alcohols. Included are ethylene vinylacetate, ethylene methyl acrylate, ethylene n-butyl acrylate, ethyleneacrylic acid, ethylene methacrylate and mixtures and blends thereof.Other examples include but are not limited to recycled polyethyleneterphthalate and polyethylene, ethylene/α-olefin interpolymers,poly-(butene-1-co-ethylene), atactic polypropylene, low densitypolyethylene, homogenous linear ethylene/α-olefin copolymers, lower meltindex n-butyl acrylate copolymers, ethylene vinyl ester copolymers).Random and block copolymers, as well as blends thereof may be used inthe practice of the invention. The polymer component will usually bepresent in an amount of from about 10% to about 60%, more preferablyfrom about 20% to about 45%, more preferably from about 25% to about35%.

Rubber based adhesive compositions and coating compositions can beprepared using random and/or block copolymers. Compositions enhanced inaccordance with the invention may be styrene-butadiene-styrene,styrene-isoprene-styrene, styrene-butadiene and styrene-isoprene blockcopolymers or mixture thereof. Preferred radiation curable rubber-basedadhesives comprise at least one high vinyl block copolymer. High vinylblock styrene-butadiene-styrene copolymers and/orstyrene-isoprene-styrene copolymers are preferred. While radial andlinear block copolymers are preferred for use, other block copolymermorphologies can be used as would be recognized by those skilled in theart. The term block copolymers include di-block, tri-block andmulti-block copolymers.

A crosslinking agent, in particular a polythiol or poly(maleimide)crosslinking agent, can optionally and desirably be used whenformulating rubber-based pressure sensitive adhesives. When formulatingrubber based UV curable adhesive compositions, the polythiol is normallypresent in a concentration of up to about 10% by weight preferably from0.3 to about 6% by weight more preferably from about 0.3 to about 1% byweight based on a total weight of the pressure-sensitive adhesive rubberand total functional polythiol. A variety of polythiols can be used,including pentaerythritoltetrathiolglycolate,pentaerythritoltetrakis(3-mercaptopropionate),trimethylolethanetrimercaptopropionate,trimethylolpropanetrithioglycolate,trimethylolpropanetris(3-mercaptopropionate) and mixtures thereof.

Radiation curable thiol-ene compositions comprise a polythiol, anolefinic or ‘ene’ compound, and the polymeric photoinitiator of theinvention. The polyene component of the thiolene system may be anycomponent containing a reactive, unsaturated group, most desirablyattached to an electron-rich atom or group. Thus, a preferred polyene isa polyfunctional vinyl ether; other suitable groups include but are notlimited to allylic ethers; vinyl sulfides; styrenes; acrylamides andacetylenes. Another suitable class of compounds are materials derivedfrom the esterification of a polyol and bicyclic enes such as norbornenecarboxylate (the reaction product of cyclopentadiene and acrylate),although their ester functionality may compromise hydrolytic stability.With respect to the thiol (mercaptan) component, primary thiolscontaining 6 to 40 carbon atoms, such as 1,10-decane dithiol, or anylinear, cyclic or branched hydrocarbon thiol containing from one to tenthiol groups may be used. Primary thiols are preferred, inasmuch as theyare most reactive, followed by secondary, and, finally, the leastreactive tertiary thiols. Examples of useful polythiols include but arenot limited to ethylene glycol bis(thioglycolate), ethylene glycolbis(β-mercaptopropionate), trimethylolpropane tris(thioglycolate),trimethylolpropane tris(β-mercaptopropionate), pentaeryth ritoltetrakis(thioglycolate, pentaerythritol tetrakis(β-mercaptopropionate),all of which are commercially available. In general, one skilled in theart can apply various methods to synthesize polythiols tailored tospecific application areas. A particularly useful class of polythiolsare derived from the esterification of various polyols withmercapto-acids such as 3-mercatopropionic acid.

The polymeric photoinitiators of the invention will typically be used inamounts of from about 0.05% by weight to about 10% by weight of thechromophore portion of the polymer based on the weight of the formulatedcomposition, preferably in amounts ranging from about 0.2% by weight toabout 3% by weight, more preferably from about 0.5% by weight to about1.5% by weight. The concentration is chosen based on the thickness ofthe application of the uncured radiation curable composition.Combinations of two or more photoinitiators may also be used to achievethe best possible cure of the formulated compositions. Photoinitiatorsare preferably used in the least amount necessary to get initiation ofcure at the line speed of the process and desired strength for the enduse contemplated. This amount will be dependent on the polymericcomposition, as well as the source of radiation, the amount of radiationreceived, the production line speed, and the thickness of the coating onthe substrate. The cure process is generally more efficient in theabsence of oxygen, for example, in the presence of nitrogen, so agreater amount of photoinitiator is generally required in the presenceof oxygen.

Adhesives and coatings comprising the polymeric photoinitiators of theinvention will be formulated with conventional additives known and usedby the skilled artisan. The polymer composition may also comprisevarious other additives chosen based upon the contemplated end use ofthe polymer. For example, if the end use is a pressure sensitiveadhesive, additives such as pasticizers, tackifiers, and fillers, whichare conventionally used in the preparation of hot melts and pressuresensitive adhesives can be added. The choice and amount of theseadditives are within the expertise of those skilled in the art.

Solid hydrogenated tackifying resins are useful in the radiation curablecomposition of the invention in concentrations ranging from about 30% byweight to about 80% by weight, preferably in amounts ranging from about45% by weight to about 70%, more preferably from about 50% by weight toabout 65% by weight. Representative tackifying resins include the C₅/C₉hydrocarbon resins, synthetic polyterpenes, rosin, rosin esters, naturalterpenes, and the like.

Waxes suitable for use in the present invention include paraffin waxes,microcrystalline waxes, high density low molecular weight polyethylenewaxes, by-product polyethylene waxes, Fischer-Tropsch waxes, oxidizedFischer-Tropsch waxes and functionalized waxes such as hydroxystearamide waxes and fatty amide waxes. It is common in the art to usethe terminology synthetic high melting point waxes to include highdensity low molecular weight polyethylene waxes, by-product polyethylenewaxes and Fischer-Tropsch waxes. Modified waxes, such as vinyl acetatemodified and maleic anhydride modified waxes may also be used. The waxcomponent is utilized at levels of greater than about 10 weight percent,typically about 20 to 40 weight percent, by weight of the adhesive.

The formulated compositions of the present invention may also compriseabout 0% by weight to about 40% by weight of an oil diluent. Suitableplasticizing or extending oils include olefin oligomers and lowmolecular weight polymers as well as vegetable and animal oil and theirderivatives. The petroleum derived oils which may be employed arerelatively high boiling materials containing only a minor proportion ofaromatic hydrocarbons (preferably less than 30% and, more particularly,less than 15% by weight of the oil). Alternatively, the oil may betotally non-aromatic. Suitable oligomers include polypropylenes,polybutenes, hydrogenated polyisoprene, hydrogenated polybutadiene, orthe like having average molecular weights between about 350 and about10,000. Examples include LUMINOL T350, a mineral oil available fromPetrocanada and KAYDOL OIL available from Witco Corporation.

Antioxidants are typically added to the commercially available compoundsin order to protect the ingredients against degradation duringpreparation and use of the compositions, however without interferingwith the irradiation curing of the polymer. Combinations of antioxidantsare often more effective due to the different mechanisms of degradationto which various polymers are subject. Examples of commerciallyavailable antioxidants include IRGANOX 1010(pentaetythrityl-tetrakis[3-(3,5-di-tert--butyl-4-hydroxyphenyl)propionate); IONOL(2,6-di-tertiary-butyl-4-methyl phenol); IONOX 330 (3,4,6-tris(3,5-di-tertiary-butyl-p-hydroxybenzyl)-1,3,5-trimethylbenzene); andPOLYGARD HR (tris-(2,4-di-tertiary-butyl-phenyl) phosphite). To ensurelong-term thermal stability, in general from about 0.1% to about 3% byweight of one or more antioxidants is included in the adhesivecompositions, preferably from about 0.4% by weight to about 1.5% byweight.

In addition to the above-described additional materials, the variouscompositions of the present invention may include other additives knownto those skilled in the art. These additives may include, but are notlimited to, pigments, fillers, fluorescent additives, flow and levelingadditives, wetting agents, surfactants, antifoaming agents, rheologymodifiers, stabilizers, photosensitizers and antioxidants. Preferredadditives are those which do not have appreciable absorption in thewavelengths of interest.

Examples of pigments and filler materials include, but are not limitedto, titanium dioxide, hydrophobic amorphous fumed silica, amorphousprecipitated silica, carbon black, and polymer powders. Examples of flowand leveling additives, wetting agents, and antifoaming agents includesilicones, hydrocarbons, fluorine-containing compounds, and non-siliconepolymers and copolymers such as copolyacrylates.

The compositions of the invention are prepared by conventional methods.As an example, the block copolymers, the tackifying resin and otherdesired components may be blended at an elevated temperature, (e.g.temperature of about 300° F.) using an extruder, a Z-blade mixer orother conventional mixing device.

The polymer composition disclosed herein can be used in mostapplications where an adhesive or coating composition is applied to abacking or substrate. The substrate can be in the form of films, tapes,sheets, panels, and the like, and can be made of materials, such as,paper, fabric, plastic, nonwoven fiber (e.g., disposable absorbentgarments), metal, foil, natural rubber, synthetic rubber, wood and woodcomposites.

Application of the composition of the invention to the substrate may beaccomplished using any conventional means, such as, roller, slotorifice, spray or extrusion coating. If a coated substrate is to be usedin the form of a roll, the back of the substrate may be coated with arelease backsize to prevent the composition from adhering to that sideof the substrate. If a substrate is to be coated on both sides androlled, a release paper or other protective means may be laid on onelayer of the composition to prevent that layer from adhering to thecomposition on the other side of the substrate. In some uses, a secondsubstrate may be applied directly to the composition.

After the polymer composition is applied to the substrate, it iscrosslinked by ultraviolet (UV) or electron beam (EB) radiation in airor nitrogen atmospheres. The crosslinking may be done immediately,during or after application of the polymer. The composition containingthe photoinitiator is exposed to ultraviolet radiation having awavelength within the range of 180 to 400 nm, preferably 200 to 390 nm,for a period of time sufficient to accomplish the desired amount ofcrosslinking. The exact length of exposure will be dependent upon thenature and intensity of the radiation, the particular ultravioletphotoinitiator and amount used, the polymer system, the thickness of thefilm, environmental factors, and the distance between the radiationsource and the adhesive film. The determination of these parameters iswithin the expertise of one skilled in the art. The actual radiationused can be actinic light from any source, provided it furnishes aneffective amount of ultraviolet radiation, since the compositions of theinvention activatable by actinic light will generally exhibit theirmaximum sensitivity to wavelengths in the ultraviolet range. Irradiationmay be carried out at any temperature, and most suitably is carried outat room temperature for economic reasons. The distance between theradiation source and adhesive on the coated substrate may range fromabout 0.32 cm to 25.4 cm (⅛^(th) to 10 inches), and preferably is from0.32 cm to 17.8 cm (⅛ to 7 inches).

The following examples are provided for illustrative purposes only.

EXAMPLES Example 1 Synthesis of 4-allyloxybenzophenone (1)

4-hydroxybenzophenone (186.7 g, 940 mmol, Fluka Chemical) was dissolvedin 2-butanone (700 mL, Fisher Scientific) in a 2 L four-necked flaskequipped with a mechanical stirrer, reflux condenser, addition funneland internal temperature probe. K₂CO₃ (195 g, 1.41 mol, AldrichChemical) was added to the reactor, and the contents was placed under aslow N₂ purge. Allyl bromide (123 mL, 1.41 mol, Aldrich Chemical) wascharged to the addition funnel. The pot temperature was raised to 65°C., at which point the allyl bromide was added over the course of 30minutes. The reaction was stirred for 6.5 h at 65° C. after the additionwas completed. At this point no starting material was present asdetermined by GC analysis.

The slurry was filtered, and the filtrate was extracted with 1% HCl_(aq)(500 mL). The organic layer was isolated, dried over MgSO₄ anhyd.,filtered, and solvent removed in vacuo to yield a pale yellow solid (1;207 g, 92%). The product exhibited acceptable ¹H and ¹³C NMR, FT-IR, GCand UV-Vis characteristics.

Example 2 Synthesis of SiH-Functional Benzophenone Derivative (2)

Allyloxybenzophenone 1 (200 g, 840 mmol) was dissolved with warming inTHF (150 mL, EM Science) and charged to an addition funnel placed on a 2L four-necked flask equipped with a mechanical stirrer, reflux condenserand internal temperature probe under a dry air purge. To the reactionvessel was added 1,1,3,3-tetramethyldisiloxane (740 mL, 4.18 mol, HanseChemie) and THF (100 mL). The internal pot temperature was raised to 50°C., at which point chlorotris(triphenylphosphine) rhodium (“Wilkinson'scatalyst”, 22 mg, 11 ppm based on mass of allyloxybenzophenone 1,Aldrich Chemcial) was added to the pot along with a portion (5 mL) ofthe 4-allyloxybenzophenone/THF solution. The internal reactiontemperature was raised to 60° C., at which point the4-allyloxybenzophenone solution was added to the reactor pot over thecourse of 45 min. The internal temperature of the reaction was heldbetween 60-65° C. during the course of the addition, which isexothermic. After the addition was complete, the reaction was stirred at60° C. for an additional 15 min., at which point no starting materialwas present by GC analysis. The reaction was allowed to cool to 35° C.,and activated carbon (3 lab scoops, Aldrich Chemical) was added. Theresulting slurry was stirred for 30 min., then filtered to yield a paleyellow solution. Solvent was removed from the product in vacuo to yielda yellow oil (329 g, 104% due to presence of ˜10 mol %1,1,3,3,5,5-hexamethyltrisiloxane in the commercial grade1,1,3,3-tetramethyldisiloxne starting material; i.e. the reaction isessentially quantitative). The product exhibited acceptable ¹H, ¹³C and²⁹Si NMR, UV-Vis and FT-IR spectral characteristics.

Example 3 Synthesis of Poly(butadiene)-Grafted Benzophenone PolymericPhotoinitiator (3)

Ricon 130 poly(butadiene) (734 g, Sartomer) was solvated in toluene(1100 mL, EM Science) in a 5 L four-necked flask equipped withmechanical stirring, reflux condenser, internal temperature probe, andaddition funnel under a purge of dry air. The SiH-functionalbenzophenone derivative 2 (328.8 g, 0.88 mol) was charged to theaddition funnel. A portion of compound 2 (˜5 mL) was added to the pBDsolution. The solution of pBD was warmed to 50° C., at which point asolution of Pt⁰ in1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (1.8 g 3.0-3.5wt. % Pt cat. soln, SIT7900.0 from Gelest) was added to the reactor pot.The internal temperature of the reaction was then raised to 80° C., andcompound 2 was added dropwise over the course of 1.5 h. An internaltemperature between 80-83° C. was maintained throughout the controllablyexothermic addition.

After the addition, the final stages of the reaction were followed byFT-IR analysis by monitoring the complete disappearance of the SiH bandat 2120 cm⁻¹. The SiH moieties were completely consumed after 30 min. at80° C. It is noted that this time may vary slightly depending on theactivity of the Pt⁰ catalyst solution.

The solution was allowed to cool to 35° C., at which point it wastreated with 7 small lab scoops of activated carbon. This slurry wasstirred for 60 minutes, then filtered. Solvent was removed from theresulting pale yellow solution in vacuo to yield a viscous pale yellowoil (1060 g, essentially quantitative chemical yield). The productpBD-bound benzophenone derivative 3 exhibited expected ¹H, ¹³C, ²⁹Si andFT-IR spectral characteristics. GPC analysis using both RI and UVdetectors (λ=320 nm) indicated the vast majority of the benzophenonechromophore was attached to the pBD backbone, and that little pBD MWchange had occurred during the grafting process (note a small MWincrease may be observed due to the now covalently bound chromophores).

Example 4 Synthesis of 2-hydroxy-3,4-dimethyl thioxanthone (4)

A solution of 2,2′-dithiobisbenzoic acid (10.0 g, 32.6 mmol, Fluka) inconc. H₂SO₄ (100 mL, EM Science) was prepared in a 250 mL three-neckedflask equipped with mechanical stirring and an internal temperatureprobe under N₂. This mixture was cooled to 5° C. on an ice bath. To thiscooled solution was added 2,3-dimethylphenol (7.98 g, 65.3 mmol, AldrichChemical) portionwise over the course of 30 min. No significant exothermwas observed. The reaction was stirred on ice for 1 h, then allowed towarm to room temperature. The reaction was subsequently heated to 65° C.internal temperature for 1 h, then allowed to cool back to roomtemperature.

Example 5 Synthesis of 2-hydroxyethyl-3,4-dimethyl thioxanthone (5)

The hydroxy-functional thioxanthone 5 (0.23 g, 0.8 mmol) was solvated inDMF (11 mL, EM Science) in a 100 mL three-necked flask equipped withmechanical stirring and reflux condenser under N₂. To this solution wasadded 2-bromoethanol (0.06 mL, 0.8 mmol, Aldrich) and K₂CO₃ (0.16 g, 1.2mmol) with stirring. The resulting slurry was heated to 90° C. on an oilbatch for 10 h.

The reaction was allowed to cool to room temperature, and poured ontodist. H₂O (150 mL). The slurry was filtered, and the isolated solidswere washed thoroughly with distilled water (3×100 mL) and once withisopropanol (20 mL, EM Science). The product was dried in a vacuum ovenand analyses by ¹H NMR and FT-IR.

Example 6 Synthesis of Mercapto-Functional Thioxanthone Derivative (6)

The hydroxy-functional thioxanthone 5 can be esterified with3-mercaptopropionic acid using standard Fisher Esterification protocol.Thus, compound 5 (1 g, 2.9 mmol) can be solvated in toluene (20 mL) in a50 mL three-necked flask equipped with magnetic stirring and aDean-Stark condenser under N₂. To this solution is added3-mercaptopropionic acid (0.3 g, 2.9 mmol, Aldrich Chemical) and acatalytic amount of p-toluenesulphonic acid monohydrate (0.006 g, 0.029mmol, Aldrich Chemical). The resulting solution can be heated to refluxto azeotropically remove water and affect esterification. When waterevolution has ceased, the solution is allowed to cool to roomtemperature and extracted with distilled H₂O (20 mL). The organic layeris isolated, dried over anhyd. MgSO₄ (Baker), filtered, and solventremoved in vacuo to produce mercapto-functional thioxanthone 6.

Example 7 Synthesis of Polymer-Bound Thioxanthone Photoinitiator (7)

The mercapto-functional thioxanthone 6 can be grafted ontopoly(butadiene) under radical conditions as described in the literature(Schapman, F.; Couvercelle, J. P.; Bunel, C. Polymer, 1998, 39(20),4955-4962.). Thus, Ricon 130 pBD (10 g) can be added to a 100 mLthree-necked flask equipped with mechanical stirring and internaltemperature probe under N₂. Compound 6 (3 g) is added, followed by2,2′-azobis(2-methylpropionnitrile) (“AIBN”, 0.013 g, Aldrich Chemical).The resulting mixture is heated to 75° C. to affect attachment of thethiol groups to the unsaturated rubber and produce the polymer-boundthioxanthone derivative 7.

Example 8 Synthesis of Amide-Linked Polymer-Bound Photoinitiator (8)

A solution of Ricon 131MA17 (28 g, Sartomer), 4-aminobenzophenone (7.8g, 39 mmol), and toluene (45 mL) were combined in a 250 mL four-neckedflask equipped with a reflux condenser, internal temperature probe, andmechanical stirring under N₂. The stirred mixture was heated to 82° C.for 7 h, then the solvent was removed in vacuo to yield the amide-linkedpolymeric photoinitiator 8 in quantitative yield. The product exhibitedacceptable ¹H, UV-Vis, and FT-IR spectral characteristics.

Example 9 UV Curable Rubber-Based PSA Composition, Curing and Evaluation

A rubber-based UV curable pressure sensitive adhesive composition wasformulated utilizing the polymeric photoinitiator described in Example3. SBS block copolymer, SB diblock copolymer, tackifier resins, oilplasticizer, and a typical stabilizer package were melt blended withvarying amounts of the polymeric photoinitiator at 300° F. Films ofvarious thicknesses were processed from this melt onto release paper,allowed to cool, and cured using a UV Process conveyor line curing unit.The unit was equipped with a 300 W medium pressure mercury arc bulb, anddose was varied by varying the belt speed of the conveyor. Doses weremeasured with an EIT Power Puck radiometer.

Cured films (and uncured reference samples) were then laminated to Mylarbacking substrate (˜2 mil substrate thickness). Sample films were cutinto a standard geometry for 180° peel and hot shear testing (1″×1″ lapbonds for hot shear testing, 1″×6″ test strips for peel testing; peeltest rate=12″/min.; hot shear conditions were 200° F. with a 500 g massattached to the sample; all sample preparation was performed undercontrolled temperature (70° F.) and humidity (50% RH) conditions).

Representative results of these tests are shown in Table 1. From thisdata several conclusions are clear. All of the samples containing thepolymeric photoinitiator are readily UV cured at typical UV doses asevidenced by the increase in hot shear strength upon UV curing. This isa result of the UV induced crosslinking of the systems which increasedtheir cohesive strength to various degrees. Hot shear strength is astandard measure of PSA temperature resistance. The increasedtemperature resistance of these adhesives allows for their use attemperatures much higher than analogous uncured PSAs. As the adhesivesare UV crosslinked, their peel strength decreases relative to theuncured reference sample. The important point with regard to this datais that peel adhesion that is acceptable for many PSA applications canbe maintained in a UV cured material while achieving excellenttemperature resistance. It is also notable that fairly thick films (5mils and thicker) of adhesive can be UV cured effectively.

TABLE 1 wt. % active chromophore 2% 1% 1% 1% 2% 2% 2% film thickness  22 2 5 2 2 5 (mil) UV Dose uncured 1000 450 1000 450 1000 1000(mJ/cm²)^(a) 180 Peel 4-5 3.6 3.8 4.8 3.1 2.3 2.8 (lb.)^(b) Hot Shear~0 >24 h 6 min 38 min 7 min >72 h 16 h (time)^(c) Failure Mode cohesivecohesive cohesive cohesive cohesive cohesive cohesive ^(a)broadband (250nm-400 nm) UV dose ^(b)1″ tape ^(c)200° F., 500 g, 1 in² lap

Example 10 Use of Polymeric H-abstraction Class Photoinitiators inThiol-ene UV Crosslinkable Systems

A prototypical thiol-ene UV curable composition was formulated utilizingthe basic resin components tetraallyl bisphenol A (Bimax), andtetrathiol 10 derived from this base resin. These two components weremixed with a 1:1 thiol:ene molar ratio. This stock resin system was thenused to produce three different formulations as given in Table 2 below.Formula (F1) is the resin system with no added photoinitiator, Formula 2(F2) is the resin system blended with 2 wt. % benzophenone (a typicalsmall molecule photoinitiator), and Formula 3 (F3) is the resin systemblended with ca. 8 wt. % of the polymeric photoinitiator of Example 3(note: 8 wt. % of the polymeric photoinitiator is equivalent to ca. 2wt. % of pendant benzophenone chromophores). The UV curing behavior ofthese formulations was evaluated by photodifferential calorimetry(Perkin Elmer DSC-7 equipped with 100 W medium pressure mercury lamp,total light intensity at sample ca. 22 W/cm²). Reactivity was judged byboth the enthalpy of polymerization (ΔH_(p)), the related chemicalconversion, and time to maximum polymerization exotherm (Δt_(o-p)).Results are summarized in Table 2 and in FIG. 1.

TABLE 2 Formulation ΔH_(p) (J/g) ~% conversion Δt_(o-p) (seconds) F1: nophotoinitiator −18 6 8.4 F2: 2 wt. % −117 40 7.8 benzophenone F3: 2 wt.% polymer- −132 45 7.8 bound benzophenone (8 wt. % polymer 3)

As can be seen by comparing formulations 1 and 2 (via Table 2 and FIG.1), the polymeric photoinitiator 3 performed as well as, if not betterthan, the typical thiol-ene photoinitiator benzophenone. Specifically,the thermodynamic quantities of enthalpy of polymerization/chemicalconversions were essentially the same. Also, the time to peak exotherm,a qualitative measure of photopolymerization kinetics, was similar forboth systems. As can be seen from formulation 1, this thiol-ene systemhas trace UV sensitivity even in the absence of any discretephotoinitiator. This frequent “initiator-free” activity of thiol-enesystems has been previously ascribed to the formation of ground orexcited state charge transfer complexes or simply the photogeneration oflow levels of radical inititiators via various routes. For the purposesof this invention, the key items of interest are the facts that 1) thesystems that incorporated added photoinitiators are orders of magnitudemore reactive than the one that did not, and 2) the polymericphotoinitiator 3 exhibits essentially equal reactivity as the smallmolecule photoinitiator benzophenone.

Example 11 Further Use of Polymeric H-abstraction Class Photoinitiatorsin Thiol-ene UV Crosslinkable Systems (Filtered Analyses)

A prototypical thiol-ene UV curable composition was formulated utilizingthe basic resin components ofpentaerythritoltetrakis(3-mercaptopropianate) (Hampshire Chemical), andtriallylisocyanurate (Aldrich, ˜200 ppm BHT stabilizer). These twocomponents were mixed with a 1:1 thiol:ene molar ratio. This stock resinsystem was then used to produce two formulations as given in Table 3below. Formula (F1) is the resin system with no added photoinitiator,Formula 2 (F2) is the resin system blended with 8 wt. % of the polymericphotoinitiator of Example 3 (note: 8 wt. % of the polymericphotoinitiator is equivalent to ca. 2 wt. % of pendant benzophenonechromophore). The UV curing behavior of these formulations was evaluatedby photodifferential calorimetry (Perkin Elmer DSC-7 equipped with 100 Wmedium pressure mercury lamp, total light intensity at sample ca. 22mW/cm²). Reactivity was judged by both the enthalpy of polymerization(ΔH_(p)), the related chemical conversion, and time to maximumpolymerization exotherm (Δt_(o-p)).

It is known to those skilled in the art that certain thiol-eneformulations exhibit significant photoreactivity without addedphotoinitiator. This formulation without added photoinitiator (F1), ifirradiated with unfiltered UV light with the spectral distribution of amedium pressure mercury arc lamp does, indeed, show such“initiator-free” activity. In this experiment, the light from themercury arc lamp was filtered with an interference filter with a ˜300 nmcutoff wavelength (i.e. light of wavelengths less than 300 nm arecompletely filtered). Results of the photoDSC experiments with a 300 nmcutoff filter are shown in Table 3 and FIG. 2. As can be seen from theenthalpy of reaction and conversion of F1 when irradiated with filteredlight, minimal reactivity is observed. This is in contrast to the sameinitiator-free formulation (F1) irradiated with unfiltered light, whichexhibits significant conversion. This unfiltered reactivity data isincluded in Table 3 under the heading “unfiltered reference F1”. As canbe clearly seen from the data on formulation F2, the polymericphotoinitator of Example 3 is an efficient initiator for this system,even in the presence of the cutoff filter. This example also clearlyshows that the bulk of this reactivity is due to the photoinitiatingcapability of the polymeric photoinitiator, as opposed to the inherentphotoactivity of this particular thiol-ene formulation. Note that thetheoretical enthlapies of reaction (for 100% conversion) for F1 and F2are different due to the different concentrations of thiol and ene inthe two formulations that results from incorporation of the polymericphotoinitiator at significant mass levels (8 wt. %). Conversely, thecalculated conversion takes this into account. The significantinitiator-free activity of this particular thiol-ene system is notable,but not useful if longer wavelengths are used for UV curing (forexample, curing through borosilicate glass). In such cases, initiatorssuch as the polymeric one utilized in this example are needed.

TABLE 3 Formulation ΔH_(p) (J/g) ~% conversion Δt_(o-p) (seconds) F1: nophotoinitiator <−63 21 6.6 F2: 2 wt. % polymer- −182 69 9.0 boundbenzophenone (8 wt. % polymer 3) unfiltered reference F1 −244 79 6.0

Example 12 Use of Hydrogen Sources as Coinitiators with PolymericH-Abstraction Class Photoinitiators

A stock rubber-based UV curable hot melt pressure sensitive adhesive(UVHMPSA) formulation was prepared using the basic components SBSrubber, hydrogentated tackifiers, saturated oil, and an antioxidantpackage. To form sample #1, 100 g of this stock PSA was blended with 9 gof the amide-linked polymeric photoinitiator 8 (ca. 2 wt. % activeamidobenzophenone chromophore). To form sample #2 a second 100 g portionof the stock PSA was added to 4.5 g of photoinitiator 8 and 1 g of ahigh MW, low odor polythiol crosslinker (ca. 1 wt. % activeamidobenzophenone chromophore and ca. 1 wt. % polythiol crosslinker).Both samples were successfully prepared by both solution and meltprocessing.

Films of both samples (nominally 2 mil dried film thickness) were drawnfrom toluene solution. Both samples were cured with a total UV dose of500 mJ/cm² on a UVProcess conveyor line (dose represents total does inthe UVA, B, C, and V regions of the spectrum as measured by a UVProcesscompact radiometer).

Dynamic Mechanical Analysis of both samples indicated that only sample#2, which contained the polythiol H-donor/crosslinker had curedefficiently. This was evidenced by the fact that the rubbery plateauregion of UV cured sample #2 extended above 150° C., well over the T_(g)of the styrene endblocks of the base rubber. Sample #1, which did notcure efficiently, exhibited a drastic reduction in elastic modulus (E′)and flow at temperatures above the styrene endblock T_(g) of ˜110° C. Itshould be noted that, in other rubber-based UVHMPSA systems,photoinitiator 8 does produce effective levels of crosslinking withoutan additional hydrogen donor/crosslinker.

Thus, in well designed formulations, the use of H-donors/crosslinkerssuch as polythiols and amines can accelerate the rate and extent of UVcure in systems incorporating the polymeric photoinitiators of thecurrent invention.

Example 13A-C Use of Polymer-Bound Pl in SIS-based Systems

Although it is known that SIS-based pressure sensitive adhesives can becrosslinked with photoinitiators, the UV dose required is excessiveunless multifunctional coupling agents are employed such as di-, tri,and tetra-functional acrylates (D. J. St. Clair, Adhesives Age, March1980, p. 30). However, and as noted by St. Clair, these formulas are notthermally stable and thus are not suitable for use in conventional hotmelt processes where manufacture of the adhesive itself will ordinarilyexpose it to several hours of high temperatures. In general, hot meltsshould show only minor property changes over 24 hours at theirapplication temperature. For instance, a viscosity change of less than25% is desirable.

Examples 13A is a comparative example of a SIS block copolymercrosslinked with a convention photoinitiator (Irgacure 819). Example 13Band Example 13C exemplify SIS block copolymers crosslinked with aphotoinitiator of the invention.

In these examples, tangent delta was measured using the followingprocedure:

A Rheometrics Dynamic Mechanical Analyzer (Model RDA 700) was used toobtain the elastic (G′) and loss (G″) moduli versus temperature. Theinstrument was controlled by Rhios software version 4.3.2. Parallelplates 8 mm in diameter and separated by a gap of about 2 mm were used.The sample was loaded and then cooled to about −100° C. and the teststarted. The program test increased the temperature at 5° C. intervalsfollowed by a soak time at each temperature of 10 seconds. Theconvection oven containing the sample was flushed continuously withnitrogen. The frequency was maintained at 10 rad/s. The initial strainat the start of the test was 0.05% (at the outer edge of the plates). Anautostrain option in the software was used to maintain an accuratelymeasurable torque throughout the test. The option was configured suchthat the maximum applied strain allowed by the software was 50%. Theautostrain program adjusted the strain at each temperature increment ifwarranted using the following procedure. If the torque was below 200g-cm the strain was increased by 25% of the current value. If the torquewas above 1200 g-cm it was decreased by 25% of the current value. Attorques between 200 and 1200 g-cm no change in strain was made at thattemperature increment. The shear storage or elastic modulus (G′) and theshear loss modulus (G″) are calculated by the software from the torqueand strain data. Their ratio, G″/G′, also known as the tan delta, wascalculated.

Example 13A Comparative Example

A pressure sensitive adhesive was formulated based on 20 parts of SISblock copolymer containing 4 parts of Quintac 3530 (available fromNippon-Zeon), 4 parts of Kraton 1119 (available from Kraton Polymers)and 12 parts of Vector 4411 (available from Dexco Polymers), 53 parts oftackifying resin (Escorez 5320, available from Exxon-Mobil ChemicalCo.), 23 parts of Britol 35T (a white mineral oil available fromCrompton), 0.6 parts of anti-oxidant (0.3 parts Irganox available fromCiba-Geigy and 0.3 Sumilizer TPD available from Sumitomo), 5 parts ofmultifunctional coupling agent (SR454 triethoxylated trimethylol propanetriacrylate) and 1 part of Irgacure 819 (phosphine oxide photoinitiatoravailable from Ciba-Geigy).

The sample was coated at 5 mils on release paper and cured under aFusion UV H bulb with a dose of 1000 mJ/cm2 of UVB.

Gel fraction in cyclohexane was determined by weighing the cured film,immersing it in a container of cyclohexane overnight, removing theswollen film in the morning and drying. Gel fraction was 21.8% or 87% oftheoretical (20% polymer plus 5% multifunctional coupling agent).Uncured films dissolve completely in cyclohexane.

RDA was conducted. Cured films exhibit tangent delta values well below1.0, preferably below 0.5, and most preferably below 0.2 at hightemperatures, i.e., beyond the styrene block Tg. Values are recorded at160-200° C., where the adhesive would be highly fluid if not forcrosslinking. Fluid materials exhibit more viscous behavior and thushigher tangent delta values. At tangent delta=1 the fluid is behavingequally viscously (fluid like) and elastically (solid like). The lowerthe value the more solid-like the behavior.

The tangent delta value of this adhesive was 0.06 at 170° C.

The viscosity of this adhesive was 2,645 cP at 275° F. Viscosityincreased linearly with time by 50% over 24 hours.

Example 13B Inventive Example of SIS-Based PSA

A pressure-sensitive adhesive was formulated based on 20 parts of SISblock copolymer (Kraton 1320 available from Kraton Polymers), 53 partsof tackifying resin (Escorez 5400, available from Exxon-Mobil ChemicalCo.), 22 parts of Britol, 0.6 parts of anti-oxidant (0.3 parts Irganox3052 and 0.3 parts Sumilizer TPD) and 4 parts of the photoinitiatordescribed in Example 3.

The adhesive was coated and UV cured as described in the above Example13A. Tan delta at 170° C. was 0.1 indicating it was well cured.

Example 13C Inventive Example of SIS-Based PSA

A pressure-sensitive adhesive was formulated based on 25 parts of SISblock copolymer (Kraton 1320 available from Kraton Polymers), 53 partsof tackifying resin (Escorez 5400), 18 parts of Britol 35T, 0.6 parts ofanti-oxidant (0.3 Irganox 3052 available from Ciba-Geigy and 0.3Sumilizer TPD available from Sumitomo) and 4 parts of the photoinitiatordescribed in Example 3.

The adhesive was coated and UV cured as described in the above Example13A. Tan delta at 170° C. was 0.03 indicating it was well cured.

Viscosity of this adhesive at 325° F. is 8575 cP. When held at thistemperature it dropped slowly by only 10% in 24 hours. A drop inviscosity upon aging at elevated temperature is typical of SIS-basedPSA's due to slow oxidative chain scission. The slight decrease inviscosity is expected and indicates no tendency of the inventivephotoinitiator to lead to premature adhesive gelation when heated.

Example 14 Comparison of Photoinitiators of Invention with ConventionalPhotoinitiators

Conventional photoinitiators (Irgacure 651 and benzophenone) and thephotoinitiator of Example 3 were used to crosslink various styrenicblock copolymers. Films were prepared using the block copolymers shownin Table 3, a photoinitiator and oil. All block copolymers were obtainedfrom Kraton Polymers. In Table 4, vinyl indicates pendant double bonds,i.e., 1,2 (SBS) or 3,4 (SIS), polymerization.

TABLE 4 high vinyl SBS SBS SIS (Kraton D1192) (Kraton D1101) (KratonD1165) % vinyl 40 10 12 % styrene 30 30 30 % diblock <1 16 16 MW (GPC)140,000 172,000 140,000

The block copolymer was used at 50 wt. %.

Irgacure 651 (available from Ciba-Geigy) was used at 0.0039 moles/100 gof solids (1 wt. %). This a standard cleavage type photoinitiator thatis known to crosslink high vinyl SBS (see U.S. Pat. No. 6,486,229 B1).

Benzophenone (available from Ciba-Geigy) was used at 0.0039 moles/100 gof solids (0.71 wt. %). Benzophenone is a standard non-cleavagephotoinitiator.

The photoinitiator of Example 3 was used at 0.0031 equivalents/100 g ofactive benzophenone sites on the polymeric photoinitiator (3.69 wt. %).

Oil (Britol 35T), the remainder, was used at 46.31 to 49% by weight ofthe solid composition.

These solid ingredients were dissolved into toluene at 50% solids andthe homogeneous solutions were used to cast films onto release liner.These were dried at 250° F. for 3 minutes to produce 2 mil dry films.These were cured as above and then tested by RDA (as described above)and for gel fraction.

Tables 5 reports the gel fraction of the polymer after cure. Table 6reports the tan delta after cure.

TABLE 5 high-vinyl SBS SBS SIS Irgacure 651 47% 63% — Benzophenone — 13%— Example 3 97% 97% 94%

TABLE 6 high-vinyl SBS SBS SIS Irgacure 651 0.27 0.25 7.6 Benzophenone0.79 0.90 8.4 Example 3 0.09 0.06 0.12

It is evident from Tables 5 and 6 that the photoinitiator of Example 3is more effective than conventional photoinitiators in curing all ofthese styrenic block copolymers and is the only one capable of curingSIS.

Many modifications and variations of this invention can be made withoutdeparting from its spirit and scope, as will be apparent to thoseskilled in the art. The specific embodiments described herein areoffered by way of example only, and the invention is to be limited onlyby the terms of the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. A sulfide-linked photoinitiator having the following structure:

wherein Q is selected from the groups consisting of poly(butadiene),styrene-b-isoprene-b-styrene (SIS), styrene-b-butadiene-b-styrene (SBS),EPDM rubber, styrene-butadiene copolymers (SBR), and acrylate polymerscontaining pendant or backbone unsaturation, Ar is selected from thegroup consisting of:

R is independently a linear or branched alkyl group, cycloalkyl,alkylenoxy, alkenyl or aryl group, an alkyl group containing aheteroatom or a carbonyl group, R² is independently a linear or branchedalkyl group, cycloalkyl, alkylenoxy, alkenyl or aryl group, an alkylgroups containing a heteroatom, a carbonyl group, H, OR, NR₂, SR, F, Cl,Br or I, X is O, NR, S, PR, or SiR₂, m=0 or 1, and y=1-100.
 2. Asulfide-linked photoinitiator formed by reacting an unsaturated compoundwith a compound having the following structure:


3. A sulfide-linked photoinitiator having the following structure:


4. A photopolymerizable composition comprising the photoinitiator ofclaim
 1. 5. A pressure sensitive adhesive comprising the photoinitiatorof claim
 1. 6. A thiol-ene photocurable composition comprising amultifunctional thiol, a multifunctional olefin, and a photoinitiator ofclaim 1.